Chemical-mechanical polishing ("CMP") processes remove material from the surface of a wafer in the production of ultra-high density integrated circuits. In a typical CMP process, a wafer is pressed against a polishing pad in the presence of a slurry under controlled chemical, pressure, velocity, and temperature conditions. The slurry solution generally contains small, abrasive particles that abrade the surface of the wafer, and chemicals that etch and/or oxidize the surface of the wafer. The polishing pad is generally a planar pad made from a porous material such as blown polyurethane. Thus, when the pad and/or the wafer moves with respect to the other, material is removed from the surface of the wafer by the abrasive particles (mechanical removal) and by the chemicals in the slurry (chemical removal).
FIG. 1 schematically illustrates a conventional CMP machine 10 with a platen 20, a wafer carrier 30, a polishing pad 40, and a slurry 44 on the polishing pad. The platen 20 has a surface 22 upon which the polishing pad 40 is positioned. A drive assembly 26 rotates the platen 20 as indicated by arrow "A". In another type of existing CMP machine, the drive assembly 26 reciprocates the platen back and forth as indicated by arrow "B". The motion of the platen 20 is imparted to the pad 40 because the polishing pad 40 frictionally engages the surface 22 of the platen 20. The wafer carrier 30 has a lower surface 32 to which a wafer 60 may be attached, or the wafer 60 may be attached to a resilient pad 34 positioned between the wafer 60 and the lower surface 32. The wafer carrier 30 may be a weighted, free-floating wafer carrier, or an actuator assembly 36 may be attached to the wafer carrier 30 to impart axial and rotational motion, as indicated by arrows "C" and "D", respectively.
In the operation of the conventional polisher 10, the wafer 60 is positioned face-downward against the polishing pad 40, and then the platen 20 and the wafer carrier 30 move relative to one another. As the face of the wafer 60 moves across the planarizing surface 42 of the polishing pad 40, the polishing pad 40 and the slurry 44 remove material from the wafer 60.
In the competitive semiconductor industry, it is highly desirable to maximize the throughput of CMP processes to produce accurate, planar surfaces as quickly as possible. The throughput of CMP processes is a function of several factors, one of which is the ability to accurately stop the CMP process at a desired endpoint. Accurately stopping the CMP process at a desired endpoint is important to maintaining a high throughput because the thickness of the dielectric layer must be within an acceptable range; if the thickness of the dielectric layer is not within an acceptable range, the wafer must be re-polished until it reaches the desired endpoint. Re-polishing a wafer, however, significantly reduces the throughput of CMP processes. Thus, it is highly desirable to stop the CMP process at the desired endpoint.
In one conventional method for determining the endpoint of the CMP process, the polishing period of one wafer in a run is estimated using the polishing rate of previous wafers in the run. The estimated polishing period for the wafer, however, may not be accurate because the polishing rate may change from one wafer to another. Thus, this method may not accurately polish the wafer to the desired endpoint.
In another method for determining the endpoint of the CMP process, the wafer is removed from the pad and wafer carrier, and then the thickness of the wafer is measured. Removing the wafer from the pad and wafer carrier, however, is time-consuming and may damage the wafer. Moreover, if the wafer is not at the desired endpoint, then even more time is required to remount the wafer to the wafer carrier for repolishing. Thus, this method generally reduces the throughput of the CMP process.
In still another method for determining the endpoint of the CMP process, a portion of the wafer is moved beyond the edge of the pad, and an interferometer directs a beam of light directly onto the exposed portion of the wafer. The wafer, however, may not be in the same reference position each time it overhangs the pad because the edge of the pad is compressible, the wafer may pivot when it overhangs the pad, and the exposed portion of the wafer may vary from one measurement to the next. Thus, this method may inaccurately measure the change in thickness of the wafer.
In light of the problems with conventional endpoint detection techniques, it would be desirable to develop an apparatus and a method for quickly and accurately measuring the change in thickness of a wafer during the CMP process.